A delay tolerant network (DTN) having a function of temporarily storing forwarding data instead of immediately forwarding it to a next hop is a network system capable of highly reliable end-to-end distribution of data contents even when the node-to-node connection is unstable and unreliable such as in a wireless ad-hoc network or satellite network.
This DTN divides forwarding data into unit sizes called bundles, and forwards each bundle to the destination. The most significant feature of the DTN is that even when the path of a bundle to be forwarded to the destination is unknown, the DTN does not discard the bundle but stores it and waits until the next hop to be forwarded is found. Even when end-to-end paths from the source to the destination do not simultaneously exist, therefore, the above operation makes it possible to forward a bundle to an intermediate node, and resume the forward of the bundle when the paths to the destination are found after that. This effectively increases the transport factor with respect to the destinations.
Accordingly, the routing method of the DTN differs from the general IP routing that finds end-to-end paths from the source to the destination.
An example of the routing method of the DTN is described in, e.g., “Anders Lindgren, Avri Doria, Olv Schelen, ‘Probabilistic Routing in Intermittently Connected Networks’, In Proceedings of The First International Workshop on Service Assurance with Partial and Intermittent Resources (SAPIR 2004); August 2004, Fortaleza, Brazil”. This reference explains a method called PROPHET. PROPHET performs routing based on the probability of distribution to the destination. More specifically, when node B is adjacent to node A, node A updates a distribution probability P_i with respect to node B byP—i=P_(i−1)+(1−P_(i−1))×P_init (0<P_init<1)  (1)where P_(i−1) is the distribution probability of node A with respect to node B before update, and P_init is an initialization constant.
If a state in which nodes A and B are not adjacent to each other continues, the distribution probability is gradually decreased as it is periodically updated by a method called aging in accordance withP—i=P_(i−1)×γ^k(0<γ<1)  (2)where γ is the number of times of the elapse of a unit time since nodes A and B are adjacent to each other lastly.
When a node has a bundle addressed to a certain node, the node exchanges the distribution probability P_i with respect to the destination node with adjacent nodes, and forwards the bundle to an adjacent node having the highest distribution probability. If the distribution probability of an adjacent node with respect to the destination node is lower than its own distribution probability with respect to the destination node, the node keeps holding the bundle without forwarding it to the adjacent node.
As described above, PROPHET performs routing by comparing the distribution probabilities with respect to the destination node calculated based on an adjacency log.